Heads up: Nope, no joy. Rufus worked OK and I copied the Win10 iso there in MBR mode, but the problem is that the target drive's three partitions - System.Reserved, C and the new partition - are all MBR and W10 will only install to GPT.
Apparently you can't change only one partition to GPT, the entire disk has to be GPT.
Or am I missing something?
I'm almost completely certain that the entire disk has to be one partitioning layout.
TBH I haven't tried to dual-boot onto the same disk in recent memory (last time for me was Win98 + NT4 I think, maybe Win98+2k). Are you manually creating a partition then telling Win10 to install onto it, or did you just point it at unpartitioned space and pressed Enter?
Win10 certainly likes to create multiple partitions, but I don't know if it has to. With MBR partitioning you're limited to 4 primary partitions IIRC. One can work around the 4 partition limit by creating IIRC a 'secondary' partition at which point logical partitions can be created inside that, but I bet that Win10 wants to be able to boot from at least the main boot partition and the recovery partition.
Looks like someone agrees with me:
I've been trying to install Ubuntu alongside windows for 2 days now I'm new to Linux and all the videos I've watched are long and tedious and they all assume you have a GPT disk. Here's the problem...
askubuntu.com
askubuntu said:
Windows 10 normally uses 3 of the 4 primary partitions of MBR leaving 1 for any other partition. However, installing Ubuntu would work fine in the last remaining partition as long as you don't create a separate Swap partition
Options that I can think of:
1 - Somehow make Win10 only use one partition (ie. skip boot and recovery partition creation), considering that you have a win7 boot partition already I don't see why this would be impossible as Win10 will take over boot management anyway.
2 - Convert win7 to GPT. I can however confirm that mbr2gpt is not part of the Win7 install (I fired up my Win7 VM and attempted to run it).
3 - Install Win10 onto a separate disk - I'm dual-booting Linux and Win10/11 since 2018 without issues (In UEFI mode, though I don't think MBR will be a problem on two disks).
This post is written first thing in the morning for me on day one of a heavy cold, I've tried my best.